Bing Maps is Microsoft's atlas web app, offering map and satellite views of earth and street view and an excellent birds eye view of cities.
Like Google Maps, Bing Maps lets you search for places, create routes and save them all with your Windows Live account. The major difference for users is that movement and zooming in and our feel different. Bing has more momentum, which feels nice, but can also result in exaggerated movements if you're not used to it.
The Bing Maps street view works well where available, and is comparable to Google's product. Where Bing excels is in its isometric bird's eye view, which offers a much more natural way to view cities instead of the traditional top down view. Its search isn't as good as Google's, however, and some map labeling is incorrect. Bing Maps' biggest flaw is that it's not included in Bing Search. Search for an address in Bing, and there is no tab for going directly to your search in Bing Maps, which is a missed opportunity.
Overall, Bing Maps is a reasonable offering, but is not quite so complete nor as well integrated as Google Maps. Hopefully the search will improve over time and the atlas will have errors ironed out.
Bing Maps looks good, but isn't quite as polished or complete as Google Maps